fair enough, but if immigration told him he needs a visa do they flag that in the system? I wouldnt risk it. Same happened to me awhile back, but luckily they let me through at the time and said next time you need a visa or you won't be let in again.
if you need a visa, then you must get it before you arrive. Otherwise you'll get a visa exempt stamp for 60 days when you arrive (assuming you have American passport?). Once you're here, you can sort out a longer-term visa option or search for a visa agency.